_

Παρασκευή, 6 Μαρτίου 2015

The GR-P ¼-wave Ground Plane Wire Antenna

GR-P as Greek Preparedness!

As title says, it is a ¼-wave groundplane for the VHF, made by wire with field expedient methods and tools. Its inspiration is the Jungle Antenna of the Vietnam era.


With some more research another reference was found in the N6CC page that has many more field expedient antenna designs.

So….Here is our antenna finished, tuned and working.


Since wire is not stout, we chose to make a frame out of string tensioned by a wooden frame (just like the Jungle Antenna), then attach the wires on the string.

Construction

We used simple materials and tools. Our object was to make a field-expedient, limited resources antenna. 


These are 4 pieces of 1mm speaker wire, a length of RG-58 coax, a electrical crimp connector, some zip ties and some heatshrink tubing.

Then we stripped the wire lengths and the coax. The coax core was connected to the wire length that would become the vertical element and the co-ax shield was twisted with the ends of the three wire lengths that would act as the antenna radials (artificial ground).


A better method would be to strip the braid out of a length of the coax, and use the core as the vertical element. It saves the hassle of joining the wire to the core.
But 2 yards of coax was what we had available at the time, which was already barely long enough, so we proceeded with adding the l/4 wire...


Connections were reinforced with heatshrink tubing and zipties. Then we waited for the field tests to resume construction.


The rest of the assembly was done in the field.


It is messy but it will all become clearer in the next step.

Building the frame.


The radiators must be set at an angle of 45 degrees for the antenna to have an impedance of 50 Ohms. For this reason a frame should be used. So we sat down with a notebook and a scientific calculator and found out the frame and rigging dimensions.
We chose to have rigging strings longer than the wire, and decided on 60cm long. That dictates frame with sides 73cm long.

And tie downs start to happen.


Finished and hoisted.


Antenna elements were attached on the strings with a jumbled mess of electrician’s tape and zipties. But it is a prototype after all! Soon we will be heatshrinking heat shrinking their whole length.

And this is a posed view with element dimensions added.


Tuning


Now here is a rookie mistake, antenna was too low over the ground.

We kept removing 3mm (1/8”) each time from the main element. When we felt we would gain no more, these were the SWR measurements.


The aim was achieving best SWR at the 145Mhz which is the lower limit of Greek HAM repeaters’ input frequenciess. But we missed it by some margin, not for overcutting, but for allowing ground effect to creep in. Foolishly the antenna was kept at a bit over 1 meter from the ground to facilitate adjustments.

When we realized this we hoisted it above wavelength-high and took the final measurements shown above.
It is off by 2MHz now, but closer to the private frequency some prepper friends are using.


Adjunct

With the occasion we also tested our other DIY ¼ ground plane, one made from single strand copper wire and a brass soldering rod (for the vertical element).
Here it is, posed on the ground.


With the occasion we also “studied” ground effect on antenna tuning.
On the ground SWR was measured at 1.37, 1-meter over was 1.25, and raised above 2m SWR fell in the expected region of 1.05. That was some lesson to be learned!
 

Δεν υπάρχουν σχόλια:

Δημοσίευση σχολίου